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4 páginas, 4 figuras, 1 tabla., Introduction Symbiotic polynoids of the genus Branchipolynoe associated with mussels are a common component of hydrothermal vent and coldwater seep communities. Recent detailed studies carried out on the Atlantic scale-worm B. aff. seepensis Pettibone, 1986 (Jollivet et al., 2000) revealed some interesting features of its reproductive biology: external sexual dimorphism, internal fertilisation and dominance of females in the population. Nevertheless, the role of symbiotic polychaetes in the communities and their relationships with their hosts remain virtually unknown. In fact, the association of the different species of Branchipolynoe with their host mytilids Bathymodiolus spp. has been usually regarded as commensalism (Pettibone, 1986; Chevaldonne et al., 1998). However the finding of bits of mussel gills in the gut of B. symmyitilida from the Galapagos Rift (Desbruyères et al., 1985) pointed to a parasitic behaviour. The present paper reports preliminary results based on material collected mainly during a recent cruise of the R/V Akademik Mstislav Keldysh (2002) at the hydrothermal vent fields along the MAR and complimentary data of previous French and Russian expeditions which allowed to better understand the true nature of the relationships between the Atlantic scale worm B. aff. seepensis with their mytilid hosts Bathymodiolus puteoserpentis and B. azoricus., This study was supported by the Russian Foundation for Basic Research (project no. 01-04- 49022).